Concurrent access to non-existing saga instances

The persister uses Unique Indexes to ensure only one document can contain the unique data.

Concurrent access to existing saga instances

The persister uses a document versioning scheme built on top of findAndModify command to atomically update the existing persisted data only if it has not been changed since it was retrieved. Since the update is atomic, it will ensure that if there are multiple simultaneous updates to a saga, only one will succeed.